MOLECULAR-STRUCTURE OF THE METHOXY-IRON(III) DERIVATIVE OF -HYDROXY-3,3'-DIAMIDOBIPHENYL)-DIPHENYL)-PORPHYRIN AND CO BINDING-PROPERTIES OF IRON(II)-PYRIDINE COMPLEXES OF 2,2'-SUBSTITUTED BIPHENYL STRAPPED PORPHYRINS

The molecular structure of the methoxy-iron(III) derivative \1(Me,OH)- Fe(OMe)\ of the strapped porphyrin (5,15-(o,o' '-hydroxy-3,3'diamidobi phenyl)-diphenyl)-porphyrin (I(Me,OH)) is reported. The 2-methyl-2'-hy droxy-substituted biphenyl unit and the porphyrin ring are covalently bound together by two amido-groups linking the 3,3'-positions of the b iphenyl strap to two ortho-positions of the two meso-phenyl rings belo nging to the 5,15-diphenylporphyrin. The iron(III) center present in \ 1(Me,OH)-Fe(OMe)\ is five-coordinate, high-spin (S=5/2). It is bonded to the four porphyrinato-nitrogens and the oxygen atom of the methoxy group. The mean-value of the four Fe-N-p distances is 2.060(4) Angstro m. The Fe-O bond length with the axial methoxy ligand is 1.867(3) A. T he porphyrin core is both domed and ruffled. The doming is small, wher eas the ruffling is more pronounced. The dihedral angle of the two phe nyl moieties belonging to the biphenyl strap lies close to 105 degrees . The orientation of the entire strap lying over the porphyrin core is probably mainly determined by a strong hydrogen bond of 2.531(5) A li nking the 2'-hydroxy group of the strap and the methoxy-oxygen bonded to the iron atom. This oxygen atom of the 2'-hydroxy strap substituent is also hydrogen bonded to the NH (N50) moiety of an amido group link ing covalently the strap to the porphyrin. The binding of carbon monox ide to five-coordinate iron(II)-pyridine complexes of such porphyrins \1(Me,OH)-Fe(py)\ and \2(Me,OH)-Fe(py)\ (py = pyridine) has been explo red. These carbonyl adducts \1(Me,OH)-(FeCO)-C-II(py)\ and \2(Me,OH)-( FeCO)-C-II(py)\ display, respectively, in THF and in KBr pellets, two v((CO)-C-12) stretching vibrations at 1966, 1944 and 1965 and 1938 cm( -1), respectively. The occurrence of two v((CO)-C-12) stretching vibra tions indicates the presence of two Fe-C-O conformers in both compound s. However, these two carbonyl adducts enriched with (CO)-C-13 display only a single (CO)-C-13 NMR signal lying at 208.1 and 208.0 ppm, resp ectively. This result indicates that the two Fe-C-O conformers present in \1(Me,OH)-Fe-II(CO) (py)\ and \2(Me,OH)(FeCO)-C-II(py) I interconv ert more rapidly than the NMR time scale. Moreover, an almost linear c orrelation is observed between the v((CO)-C-12) stretching vibrations and the C-13 chemical shifts of the (CO)-C-13 NMR signals in a series of iron(II) carbonyl adducts in which the second axial ligand of iron is pyridine, \1(Me,OH)Fe((CO)-C-13) (py)\ and \2(X,Y)-Fe((CO)-C-13) (p y)\[(X,Y)=(Me,OH), (Me,OMe), (H,H), (Me,Me), (OMe,OMe) and (NO2,NO2)]. In these carbonyl adducts, the v((CO)-C-12) stretching vibrations vary between 1938 and 1987 cm(-1) and the chemical shifts delta(C-13) Of t he C-13 NMR signals range from 202.5 to 208.1 ppm (versus TMS). Values of delta(C-13) as large or larger than 208 ppm are observed in myoglo bins and hemoglobins of various origins and in peroxidases in which th e second axial ligand of iron is a histidine. Although an involvement of a hydrogen bond between a distal histidine residue and the coordina ted CO molecule in the large chemical shifts of the (CO)-C-13 NMR sign als observed in these hemeproteins has been excluded, our model compou nds, in which the axial ligand trans to CO is held constant, seem to i ndicate that the polarity of the 2'-OH biphenyl substituent of the por phyrin strap plays a role in the large chemical shifts of the (CO)-C-1 3 NMR signals observed in II(Me,OH)-Fe((CO)-C-13) (py) I and \2(Me,OH) -Fe((CO)-C-13)(py)\. (C) 1998 Elsevier Science S.A. All rights reserv ed.